This invention relates generally to a ballast filter and, more particularly, to a ballast filtering scheme for reduced harmonic distortion.
Total harmonic distortion (THD) is a term which reflects and can be calculated to determine the amount of harmonic distortion present in current supplied to a ballast from a utility power line. Electronic ballasts are well known for drawing line current rich in harmonic content. Filters are provided at the front end of an electronic ballast to limit the harmonics drawn from the utility power line.
Many conventional electronic ballasts operate at a THD ranging from about 14% to 18% through the use of an LC low pass filter. Demands recently imposed by the utility industry as well as by the general public typically require the line current THD drawn by the ballast to be less than 10%.
In meeting this lower THD level, a conventional LC filter must increase its low pass attenuating characteristics at the sacrifice of efficiency, that is, by consuming far more power.
It is therefore desirable to provide an improved ballast filtering scheme in which the THD can be reduced while maintaining a relatively high ballast efficiency, that is, without a substantial increase in power dissipation of the ballast filter.